1. Field of the Invention
The present invention relates to a magnetic recording/reproducing apparatus and particularly relates to a helical scan type video tape recorder (VTR) having at least one rotary erase head and having an edit function. More specifically, the present invention relates to the helical scan type video tape recorder (VTR) which prevents a fringe effect caused by the erase head from affecting other tracks than cut-in and cut-out points.
2. Description of the Prior Art
A conventional magnetic recording/reproducing apparatus in which a rotary erase head is mounted and for which an edit function is provided is illustrated in FIGS. 1 and 2.
As shown in FIG. 1, a pair of record/playback heads (i.e., A head and B head) 2A, 2B are mounted with a mutual angular interval of 180.degree. on a rotary drum 1. Two rotary erase heads 3A and 3B are respectively mounted at angular positions 130.degree. advanced from the record/playback heads 2A, 2B with respect to a rotational direction denoted by a1 of the rotary drum 1.
A magnetic tape 4 is threaded over the outer periphery of a rotary drum 1 and runs in the direction denoted by the arrow a2. The magnetic tape 4 approximately covers a predetermined wrap angle equal to the spacing between the record/playback heads 2A, 2B.
The record/playback heads 2A and 2B are oriented so as to slant across the magnetic tape 4 so as to form record tracks 5A, 5B adjoining each other alternatingly. The record/playback heads 2A, 2B are slanted +7 degrees or -7 degrees of azimuth angles to form the record tracks in a guardbandless format.
On the other hand, the rotary erase heads 3A and 3B are slanted -20 degrees and +20 degrees of azimuth angles, respectively. The rotary erase heads 3A and 3B are mounted at positions offset vertically with respect to the record/playback heads 2A and 2B to an extent corresponding to two tracks. When the two record/playback heads 2A and 2B are recording, the previously recorded signals on the record tracks 5A and 5B are first erased by the rotary erase heads 3A and 3B.
In the construction of the conventional magnetic record/reproduction apparatus, the A and B heads operate alternatingly in response to head switchover signals CH.sub.AB (refer to FIG. 3(A)). When insertion editing is carried out, recording currents I.sub.RECA and I.sub.RECB (refer to FIG. 3(C) and FIG. 3(D)), are conducted through the A and B heads (, i.e., record/playback heads 2A, 2B) with an insertion edit interval T.sub.INS covering an integral number of tracks (tracks on which one video frame is recorded). On the other hand, an erase current I.sub.FE (refer to FIG. 3(B)) is conducted through the two rotary erase heads 3A and 3B over the entire insertion editing interval T.sub.INS.
In the conventional magnetic recording/reproducing apparatus shown in FIG. 1 and FIG. 2, the erase current through the rotary erase heads 3A and 3B is about twice as great as the record current so that erasure of low-frequency information components recorded deep in the magnetic tape can be assured.
However, since the erase current is relatively strong, magnetic flux "leaks" to adjacent tracks when the magnetic flux generated from the rotary erase heads 3A and 3B is transmitted toward the record tracks 5A and 5B. This phenomenon (referred to as cross-talk or fringe effect) partially erase fringe portions of tracks adjacent to the record tracks 5A and 5B being erased.
This fringe effect deteriorates the picture quality of the previously recorded video signals at the editing start point (referred to as the cut-in point) and at the editing end point (referred to as the cut-out point) when the insertion editing operation is carried out on magnetic tape 4 on which video information is pre-recorded. That is to say, when the insertion editing operation is carried out in such a way that the recorded signal ranging from a track TC.sub.IN at the cut-in point to a track TC.sub.OUT at the cut-out point is updated with an insertion signal, e.g., a commercial, an unnecessarily erased part ERA due to the fringe effect of the rotary erase head 3A will appear on the outside of the edge of the cut-in point track TC.sub.IN and an unnecessarily erased part ERB due to the fringe effect of the rotary erase head 3B will also appear on the outside of the edge of the cut-out track TC.sub.OUT. It should be noted that the fringe effect on the tracks between the cut-in track TC.sub.IN and the cut-out track TC.sub.OUT does not constitute a practical inconvenience since the newly recorded signal overlaps on the original pre-recorded signal.
When the insertion editing operation is carried out in this way, the picture quality of the video signals recorded on the track prior to the cut-in point track TC.sub.IN and on the track subsequent to the cut-out point track TC.sub.OUT is lowered. For example, when still reproduction for these tracks is carried out, only video images of picture quality lowered to a practically impermissible degree can be reproduced.